(1) Field of the Invention
The present invention relates to improved compositions for use in battery systems. The improved compositions comprise a dry mixture of sodium hydroxide and sodium oxide which when activated by water form heated electrolyte solutions.
(2) Prior Art
Battery cells having liquid electrolytes are known in the art. U.S. Pat. No. 3,440,099 to Okinaka and U.S. Pat. No. 5,215,836 to Eisenberg illustrate two such systems. The Okinaka patent relates to the improvement of the charging characteristics of nickel-cadmium cells by the addition of an additive selected from the group consisting of sodium borate, potassium borate, sodium phosphate, potassium phosphate and mixtures thereof to an aqueous electrolyte solution. Okinaka employs the electrolyte solution with the addition to achieve rechargeable, i.e. secondary battery, results. Okinaka does not concern primary batteries and/or circulating electrolytes.
The Eisenberg patent relates to a battery having a zinc or zinc alloy anode, a metal oxide or hydroxide cathode and an alkaline electrolyte comprising a solution of a salt formed by the reaction of boric acid, phosphoric acid or arsenic acid with am alkali or earth alkali hydroxide present in a sufficient amount to produce a stoichiometric excess of hydroxide to acid in the range of 2.5 to 11.0 equivalents per liter, and of a soluble alkali or earth or earth alkali fluoride in an amount corresponding to a concentration range of 0.01 to 1.0 equivalents per liter of total solution. Eisenberg reacts an acid and a base, which could not be stored together, to produce a new neutral salt electrolyte. Eisenberg is not directed to a primary battery and/or circulating electrolytes.
FIG. 1 illustrates a prior art aluminum-silver oxide battery system 10. Such a battery system has been proposed and developed to power an assortment of underwater vehicles. In this system, the stored dry battery is activated by the introduction of an electrolyte solution of alkali metal hydroxide, i.e., sodium hydroxide, dissolved in sea water or fresh water. A majority of the presently considered applications will use sea water. The battery system 10 produces electrical energy with the introduction of the electrolyte, while undergoing the following electrochemical reaction: EQU 2Al+3AgO+2NaOH.fwdarw.2NaAlO.sub.2 +3Ag+H.sub.2 O (1)
It is believed that to a lesser degree, a corrosion reaction also takes place. The following is the corrosion reaction which is believed to take place: EQU 2Al+H.sub.2 O+NaOH.fwdarw.2NaAlO.sub.2 +3/2H.sub.2 (2)
During storage, the entire system 10 is in a dry state under an atmosphere of an inert gas such as dry nitrogen or dry argon. Referring now to FIG. 1, the system 10 includes a battery cartridge 12 which typically consists of a number of bipolar assemblies of aluminum anodes and silver peroxide cathodes separated by a suitable thin metal foil, i.e. foil made from copper, nickel or silver. The bipolar assemblies are arranged under axial compression in a pile configuration in which the anode of one bipolar assembly and the cathode of an adjacent bipolar assembly form and constitute an electrochemical cell. The number of such assemblies in a given battery design is determined by the voltage requirement placed on the battery. Thus, the battery cartridge 12 is assembled and stored in the dry charged state.
The battery cartridge 12 can not perform its designed role as a battery in the dry state. It must be activated by flooding it with a hot, alkaline, aqueous solution referred to as an electrolyte. In an aluminum-silver oxide battery, the electrolyte is typically in the range of 11 to 17% by weight sodium hydroxide (NaOH) in solution at temperatures between 130.degree. F. and 220.degree. F. The higher concentrations and temperatures are frequently used for higher rate discharges of the battery (above 3 amperes per square inch of electrode area in a cell) and lower concentrations and temperatures are frequently used for lower rate discharges of the battery (below 3 amperes per square inch of electrode area in a cell). Additives may also be included in the electrolyte solution for specific purposes. For example, the electrolyte may contain about one percent by weight sodium stannate (Na.sub.2 SnO.sub.3. 3H.sub.2 O) as a corrosion inhibitor for the aluminum anode.
In these types of batteries, there is usually a requirement for fast activation of the battery. For a high rate of discharge, fast activation implies the rapid dissolution of the NaOH to a temperature on the order of 200.degree. F.
Referring again to FIG. 1, a typical scenario for battery activation involved initiation by the introduction of water or sea water into the battery system 10 through a flow valve 16. The water or sea water passes through or around a circulating pump 18 and a flow valve 20 to a circulating electrolyte reservoir 22 where a charge of dry NaOH is stored. The water or sea water entering the electrolyte reservoir 22 causes sufficient turbulence to enhance the dissolution of the NaOH in a few seconds, provided the NaOH is in a suitable geometric form, such as small beads or powder. The electrolyte so formed should be of the correct concentration, which is a nominal 15% by weight. The temperature of the electrolyte may rise 80.degree. F. to 100.degree. F. (depending on the final concentration) above the temperature of the incoming water or sea water by virtue of the heat of solution of the NaOH. The resulting temperature may still be too low to operate the battery at high rate, however, a temperature as low as 130.degree. F. would be sufficient to allow low to medium rate discharge of the battery. The electrolyte travels from the circulating electrolyte reservoir 22 through a flow valve 24 (initially configured to by-pass a heat exchanger 26) and via a heat exchanger by-pass 28 to enter and fill the cells of the battery cartridge 12. The inert gas, under which the system 10 was stored, having been forced ahead of the incoming rush of liquid electrolyte, and any hydrogen generated as the battery cartridge 12 was being filled with electrolyte, are both separated from the liquid electrolyte by means of a gas-liquid separator 30 and dispensed overboard into the sea. The degassed electrolyte then exits the gas-liquid separator 30, and after passing through the flow valve 16 (now configured to stop the influx of water or sea water), re-enters the circulating pump 18 which is now operating from battery power from the battery cartridge 12. The re-circulation of the electrolyte around the electrolyte flow path of the battery system 10 and through the cells of the battery cartridge 12 is caused by the impulse pressure of the circulating pump 18 such that the electrolyte in the battery cartridge 12 is changed about 30 times a minute, depending on the rate of discharge. With each complete pass through the battery cartridge 12, the electrolyte temperature rises about 20.degree. F. to the point (above 200.degree. F.) where the battery can be discharged efficiently at high rate. Depending upon the volume of the battery system, the temperature of the water or sea water, and the final concentration of the NaOH solution, the time elapsed from the activation of the battery to the time the electrolyte reaches 200.degree. F. can be on the order of 30 or 40 seconds. As the temperature reaches a selected value, some of the hot electrolyte leaving the circulating electrolyte reservoir 22 is diverted through the heat exchanger 26 for cooling with the heat being transferred to the source of the water such as the sea. The selected value depends upon the desired rate at which the battery system will generate energy. Upon leaving the heat exchanger 26, the electrolyte rejoins the electrolyte stream in the by-pass 28 so that the temperature of the electrolyte entering the battery cartridge 12 is correct for the amount of energy being withdrawn from the cartridge 12.
Variations on the above scheme include activating the battery such that a higher concentration of NaOH is formed, perhaps as high as 30 percent by weight. This allows a higher temperature to be reached during battery activation. It also allows for dilution of the electrolyte in order to reduce the concentration of the detrimental contaminant, sodium aluminate (NaAlO.sub.2), while maintaining a NaOH concentration above 15% by weight. This scheme requires configuring the flow valve 20 so as to allow a small portion of the electrolyte passing through it to exit the system 10 over board as waste electrolyte. At the same time, the valve 16 must be configured to allow small amounts of water or sea water to enter the electrolyte stream so as to keep the system liquid volume constant.
FIG. 2 illustrates yet another known battery system 10'. In this system, the circulating electrolyte reservoir 22 is kept small to minimize the length of time required to fill the battery cartridge 12 and to minimize the length of time required for the waste heat of the battery to increase the temperature to that required for high rate discharge. In the space saved by making the circulating electrolyte reservoir 22 smaller, an electrolyte replenishment reservoir 34 and a metering pump 36 are installed. The other components of the system 10 ' are identical to the system of FIG. 1. The electrolyte replenishment reservoir 34, which is isolated from the circulating electrolyte, can be filled with water or sea water while, and after, the battery has been activated as described above. The replenishment reservoir 34 preferably contains sufficient NaOH in a dry state to produce a more concentrated solution, on the order of 60% by weight NaOH, in the reservoir 22 and through the cells of the battery cartridge 12. Higher solution concentrations (greater than 60 percent by weight) are not practical at this point due to the high temperature that are needed for rapid solution of the NaOH at higher concentrations. It should be noted that at higher concentrations, the heat of solution of NaOH decreases. Specifically, at a concentration of 75% by weight, the heat of solution becomes as little as one-fifth of the heat produced at 42% concentration. Also, the concentrated NaOH solution, on the order of 60%, reduces the amount of water or sea water needed on board the underwater vehicle, and hence reduces the overall weight of the underwater vehicle. During the operation of the battery system 10' shown in FIG. 2, when it becomes necessary to discard some waste electrolyte through the flow valve 20, an appropriate amount of concentrated NaOH (60%) solution is introduced into the circulating electrolyte reservoir 22 by means of the metering pump 36. At the same time, some dilution water or sea water is brought into the electrolyte via the flow valve 16.
While the above discussion focuses on aluminum-silver oxide batteries, the same scenario applies to aluminum-hydrogen peroxide batteries and similar alkaline, aqueous batteries. The only change that is typically made is to the concentration and the temperature of NaOH that is stored in the reservoir 22 and thereby circulates throughout the battery system.
While the above mentioned systems are workable, there are some drawbacks which limit mission capability. These drawbacks include (a) the length of time to fill the circulating electrolyte reservoir, (b) having to wait for the waste battery heat to raise the electrolyte temperature to the point that will allow the battery to be discharged at a high rate, and (c) the weight and volume penalty of having a circulating electrolyte reservoir 22 or replenishment electrolyte reservoir 34 of sufficient size to supply all the NaOH, at the correct temperature and concentration, required for the entire mission. The present invention minimizes these drawbacks.